Photo electric apparatus



Sept. 17, 1935. SHEPARD, JR 2,014,786

PHOTO ELECTRIC APPARATUS Filed Oct. s, 1935 lllllb QAAA vvvv L l l l INVENTOR FRANCIS A. SHEPARD. JR.

ATTORN EY Patented Sept. 17, 1935 PATENT OFFICE PHOTO ELECTRIC APPARATUS Francis H. Shepard, Jr., Rutherford, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 3, 1933, Serial No. 691,919

6 Claims.

My invention relates to photoelectric apparatus and more particularly to means for operating photoelectric tubes at maximum efficiency and sensitivity.

The conventional gas filled phototube having a p hotocathode and an anode operates at maximum efficiency and is most sensitive when the voltage across the tube is the maximum the tube will stand without glowing. Glowing causes rapid deterioration of the phototube cathode. In practice, a load resistor is connected in series with the phototube and a source of voltage for the tube. In order to prevent the voltage across the tube from rising above the maximum allowable voltage of the tube when the light is taken off the tube or when the intensity of the light on the tube decreases with a resultant decrease in phototube current, the voltage across the tube and the load resistor must be limited to some value less than the maximum operating voltage which the tube can stand without glowing. The reason for so limiting the voltage is that as the photocurrent decreases the voltage drop through the load resistor decreases,while the voltage drop across the tube increases, the effective resistance of the phototube increasing as the intensity of the light on the tube decreases. With no light on the tube no current would pass through the tube and resistor and the entire voltage drop from the voltage supply would be across the tube which voltage, if it exceeded the maximum allowable operating voltage, would cause the tube to glow. The voltage across the tubes is, therefore, of necessity considerably less than the maximum allowable operating voltage to insure against glowing of the tube, hence, the tube is not ordinarily operated at maximum efficiency or sensitivity.

The principal object of my invention is to provide means for operating a phototube at its optimum sensitivity and efiiciency without danger of causing a glow discharge through the tube or otherwise adversely affecting the tube due to a voltage increase across the tube above the maximum allowable operating voltage when the photocurrent decreases.

The novel features which I believe to be characteristic of my invention are set forth with particularity in theappended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

The figure shown in the drawing is a diagram?- matic representation of a phototube and asso ciated circuit embodying my invention.

Referring to the drawing, the phototube l has an anode ll and a photocathode l2 connected to the negative pole of the voltage supply source, preferably a battery {3, although any other source of direct current voltage supply would be 5 satisfactory, the positive pole of the battery being connected to the anode H in series with the variable load resistor M which has a high value of resistance of the same order as the internal phototube direct current resistance.

In accordance with my invention I associate with the phototube a thermionic voltage regulator such as a thermionic device I 5 having an electron emitting cathode electrode 16, a grid electrode I I connected to the anode ll of the photocell, and a plate electrode [8 connected to the positive pole of the battery I3 through a plate circuit having in it an electro-responsive operating means, such as an electro-magnetic relay for controlling a circuit 2| although this electro- 20 responsive operating means could be any other form of electro-responsive device such as a meter or the input of an amplifier. The cathode i6 is connected to the battery I 3 through a voltage divider IS. The voltage regulator I5, which by way of example has been shown as a thermionic discharge device, may for the purpose of my invention take the form of any variable impedance the value of which decreases upon the applicationof a potential above a certain predetermined value.

In operation the slider on the voltage divider I9 is set so that the voltage impressed upon the cathode l6 from the battery l3 has a value slightly less than the maximum allowable voltage which the phototube l0 can stand without glowing. The slider on the load resistor M is set so that the current flowing through the load resistor l4 and the phototube ID, at the particular light intensity with which the phototube is to operate, will cause a voltage drop across the tube which is slightly less than the voltage impressed on the cathode l6. Due to this voltage drop the grid I1 is biased slightly negative with respect to the cathode l6 so that at the opera-ting voltage with the normal photocurrent through the phototube l0 no current will flow through the grid circuit of the thermionic device l5. For example, if the voltage which is impressed on the cathode is 90 volts, the drop across the phototube is adjusted to 88 volts, thus making th d 2 avolts negative with respect to the cathode.

As thelight intensity on the cell decreases the effective resistance of the phototube increases with the result that the photocurrent decreases.

With a decrease in photocurrent the voltage drop across the phototube increases, causing the voltage on the grid to increase in a positive direction. A point is quickly reached where the grid becomes positive with respect to the cathode, whereupon a grid current flows. The constants of the associated elements in the circuit are such that the increase in grid current through the thermionic device together with the current through the phototube is always sufficient to maintain the voltage drop through the load resistor M at such a value that the voltage drop across the phototube is prevented from rising above the maximum allowable operating voltage. In this way the phototube is prevented from glowing on decrease of the phototube current for any cause, thus permitting operation of the phototube at its maximum efficiency and sensi tivity.

The change in the anode cathode current through the thermionic device l5 when the grid changes its potential can be used to operate an electro-responsive device; for example the electro-magnetic relay 20, to control a circuit 2| of some kind. For the purposes of this invention the thermionic device l5 could obviously be a two electrode thermionic device with the amplifying or operating circuit associated with the phototube connected across the load resistor l4.

While I have indicated the preferred embodiment of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations maybe made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:-

1. The combination with a gas filled phototube having a photocathode and an anode, a source of voltage supply of its negative terminal connected to said photocathode, and a resistor connected between said anode and the positive terminal of said source of voltage supply having at normal photo current through the tube a voltage drop such that the voltage drop across said tube is the maximum allowable operating voltage without causing said tube to glow, and a thermionic voltage regulator connected between the anode of said phototube and a less positive point on said source of voltage to permit in response to an increase in voltage drop across said phototube a suflicient increase in thermionic current through said regulator to maintain the voltage drop across said resistor and thereby prevent the voltage drop across said phototube rising above the maximum allowable operating voltage.

2. A circuit for operating a gas filled phototube at substantially maximum sensitivity including a phototube having a cathode and an anode, a source of voltage supply for said phototube, a resistor connected between the anode of said phototube and the positive pole of said voltage supply, a connection from said cathode to the negative pole or said source, and a thermionic voltage regulator connected between said anode and a point of a potential intermediate the potentials of said poles of said voltage supply.

3. A circuit for operating a gas filled phototube at substantially maximum sensitivity including a phototube having an anode and a photocathode, a source 01 voltage supply for said phototube, a connection between the photocathode and a point of relatively negative potential on said source, a resistor connected in series with said tube between said anode and a point of positive potential on said source of voltage supply to provide the maximum allowable volt age drop across said tube with a predetermined light intensity without causing said tube to glow, anelectron discharge device having a cold electrode connected to the anode of said phototube and a thermionic cathode cooperating with said cold electrode, and a connection between said thermionic cathode and a point on said source of voltage supply of a potential intermediate said positive and negative potentials for maintaining said thermionic cathode at a bias which is positive with respect to said cold electrode when the current through said phototube and resistor is above a predetermined limit thereby preventing said electron discharge device drawing current through said resistor, and negative with respect to said cold electrode when the photocurrent through said tube decreases below a precletermined limit whereby said electron discharge device will draw sufficient current through said resistor to maintain the voltage drop across said resistor and thereby maintain substantially con stant the voltage impressed upon said phototube.

4. In combination a gas filled phototube having a photocathode and an anode, a source of voltage supply for said phototube, a resistor connected in series with said tube between said anode and said source of voltage supply to provide the maximum allowable operating voltage across said tube without causing said tube to glow, a regulator comprising a thermionic device having a thermionic cathode, a grid connected to said anode and a plate connected to said source of voltage, current responsive means connected in circuit with the plate of said thermionic device, and means connected between said thermionic cathode and said voltage supply for biasing said cathode positive with respect to said grid during normal operation of said phototube, said grid becoming positive with respect to said cathode in response to a decrease of the phototube cur rent thereby providing a grid current through said resistor sufficient to maintain the voltage drop across said resistor and prevent the voltage applied to said phototube rising above the maximum allowable operating voltage.

5. The combination with a phototube having a photocathode and an anode, a source of voltage with its negative terminal directly connected to said photocathode, and a resistor connected between said anode and the positive terminal of said source of voltage, a voltage regulator for said phototube comprising a thermionic discharge device with an electron emitting electrode and a cold electrode, a connection from said cold electrode to the anode of the phototube, and a connection from said emitting electrode to a positive source of voltage of a value slightly greater than the normal or static potential drop across said phototube.

6. The combination w th a phototube having a photocathode and an anode, a source of voltage in series with a resistor connected across the phototube for applying a normal operating voltage to the electrodes of the phototube, and an impedance connected across said phototube which decreases in value in response to a rise in voltage above said operating voltage.

FRANCIS H. SHEPARD, JR. 

